A system provided with a catalyst in an exhaust system of an engine for purging exhaust gas is widely known. As purging performance of the catalyst is deteriorated by long-term use, various methods for detecting the deterioration are proposed.
A patent literature 1 discloses technique for executing active AFR control that transition from leanness to richness and transition from richness to leanness as to AFR of exhaust gas flowing into the catalyst are alternately repeated, integrating oxygen purge amounts on the basis of transition of the AFR of exhaust gas flowing into the catalyst, an exhaust flow rate and others in the transition to richness, integrating oxygen storage amounts on the basis of transition of the AFR of exhaust gas flowing into the catalyst, an exhaust flow rate and others in the transition to leanness and making catalyst deterioration diagnosis on the basis of each integrated value in a catalyst deterioration determination system that determines deterioration of an exhaust purge catalyst provided with an oxygen storage facility.